This invention relates to a scroll compressor, and more particularly to a structure with variable action area on which a discharge back pressure is acting to prevent an axial leakage between the tips of scroll wraps and the bottom of the opposing scroll and to prevent abrasion of the scroll wrap in the scroll compressor.
In general, the conventional scroll compressor comprises a fixed scroll 10, an orbiting scroll 11, an Oldham coupling 12 as a device for preventing rotation, a main frame 13, and a crankshaft 14, as shown in FIG. 1. The fixed scroll 10 is fixed to the main frame 13, so as to be movable axially, by a leaf spring 15 (see FIG. 5) and a bolt. As the orbiting scroll 11 is connected to the crankshaft 14, the orbiting scroll 11 is orbited by a motor which comprises a rotor 16 and a stator 17. However, rotation of the orbiting scroll 11 is otherwise prevented by Oldham coupling 12.
A suction port 18 is formed under the orbiting scroll 11, and a discharge port 19 is formed at the center of the fixed scroll 10. A closer 20 (FIG. 2), a back pressure chamber 30 (FIG. 5), and a back pressure hole 31 (FIG. 5) are formed in an upper portion of the fixed scroll 10, and a discharge chamber 32 is formed as illustrated.
The refrigerant gas drawn into the scroll compressor through the suction port 18 is sucked in the fixed and orbiting scroll by the orbiting motion of the orbiting scroll 11 and, at the same time, is trapped in lunette-shaped pockets. By the continuous orbiting motion of the orbiting scroll 11, the refrigerant gas within the trapped volume is continuously moved towards the center of the wraps while being reduced in volume until it is discharged to the discharge port 19. Generally, the crankshaft 14 is rotated 2 to 3 times during one cycle from suction to discharge.
During the process of compression in the scroll compressor, there is some leakage of the refrigerant gas from the high pressure inner pocket to the low pressure outer pocket. Axial leakage takes place in a gap between the tips of the scroll wraps and the bottom of the opposing scroll, and radial leakage occurs in a gap between the opposing scroll wraps.
In order to prevent axial leakage the conventional scroll compressor, as shown in FIG. 2, has the fixed scroll 10 fixed to main frame 13 (FIG. 1) so as to be axially movable. The discharge pressure acts on the fixed scroll 10 from the upper side, so that any gap between the tips of the scroll wraps and the bottom of the opposing scroll becomes tighter so as to minimize the axial leakage.
FIG. 3 is a pressure distribution diagram of the conventional scroll compressor. A downward sealing force may be written as EQU .SIGMA.F=F.sub.d +F.sub.s1 -F.sub.S2 -F.sub.c ( 1),
where F.sub.d is a sealing force by the discharge pressure, F.sub.s1 is a sealing force by the low pressure of the refrigerant gas filled in the shell of the compressor, F.sub.s2 is a low pressure, and F.sub.c is a repulsive force caused by a compressive force. And, F=P.times.A, where P is a pressure and A is a area.
As shown in the above equation, since the intensity of the force acting on the fixed scroll 10 is determined by that of the discharge pressure, the scroll compressor should be designed to get the best sealing effect according to any driving condition, that is, the standard condition.
FIG. 4 indicates the totality of conditions (#1.about.#18) within which the scroll compressor may be operated. In the above-mentioned scroll compressor, however, the difference of the sealing forces is too large over the range of respective operating conditions, and the sealing force may be negative at some operating conditions. A negative sealing force means that the fixed scroll 10 is pushed upwardly, which indicates that the desired compression is not obtained because of axial leakage. On the other hand, when the sealing force is large, the gap between the tips of the scroll wraps and the bottom of the opposing scroll becomes too small and, because of this, the motor may be overloaded. Further, if the sealing force is larger than a certain limit, the tips of the scroll wraps become worn.
FIG. 5 indicates another type of conventional scroll compressor.
In this type of conventional scroll compressor, a back pressure chamber 30 with a uniform cross section is formed on the upper surface of the fixed scroll 10 and a part of the refrigerant gas is sent to the back pressure chamber 30, of which the pressure is uniformly maintained, through a back pressure hole 31 for preventing axial leakage and abrasion of the tips caused by too much sealing force. Therefore, a uniform sealing force acts on the upper surface of the fixed scroll 10 so that the axial gap is minimized, thus minimizing axial leakage.
In the case where back pressure is used so as to minimize axial leakage as shown in FIG. 5, however, a part of the refrigerant gas during compression flows into the back pressure chamber. Thus a loss of efficiency occurs in the P-V diagram as illustrated in FIG. 6. In other words, there is a problem in that the hole between the discharge chamber and the back pressure chamber causes low efficiency of the scroll compressor.
FIG. 7 indicates the pressure distribution diagram of this conventional scroll compressor. Therefore, the downward sealing force may be expressed as EQU .SIGMA.F=F.sub.d+Fb +F.sub.s1 -F.sub.s2 -F.sub.c ( 2),
where F.sub.d is the sealing force by the discharge pressure, F.sub.b is a sealing force by the back pressure, F.sub.s1 is the sealing force by the low pressure of the refrigerant gas filled in the shell of the compressor, F.sub.s2 is the low pressure, and F.sub.c is the repulsive force caused by the compressive force.
Consequently, conventional scroll compressors which use the discharge pressure for sealing have a problem of a drop in efficiency caused by leakage of the refrigerant gas. And, they also experience faster wear caused by abrasion of the tips of the scroll wraps due to too large a downward sealing force which may at times occur.